Cyclone for separating fine solid particles from a gas stream

ABSTRACT

A novel cyclone is disclosed that is effective for separating, from a contaminated gas stream, solid particulates having diameters as low as 4-5 microns. When multiple cyclones of the present invention are affixed between upper and lower tube sheets in a separator device, fine particle removal is possible to the extent required 1) by stringent regulations governing particulate emissions into the atmosphere, or 2) to prevent damage to turbine blades in downstream power recovery equipment. The cyclones are especially relevant to the problem of removing catalyst fines from refinery effluents, most notably fluid catalytic cracking (FCC) regenerator flue gas. The cyclone separation efficiency is enhanced through the use of 1) a uni-directional flow of gas from the contaminated gas inlet to the clean gas outlet and 2) discharge openings on the surface of the cyclone body that allow ejection of solid particulates.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority from Provisional application Ser. No.60/208,557 filed Jun. 2, 2000, the contents of which are herebyincorporated by reference.

FIELD OF THE INVENTION

The present invention relates to a novel cyclone separator for removingfine solid particulates from a gas stream. The cyclone is especiallyapplicable in a third stage separator apparatus, often used to purifythe catalyst fines-laden flue gas stream exiting a refinery fluidcatalytic cracking (FCC) catalyst regenerator.

BACKGROUND OF THE INVENTION

The emission of particulates in industrial gas streams must be carefullycontrolled in light of federal, state, and local regulations designed tocurtail pollution. In the area of oil refinery operations, a majorconcern regarding particulate emissions lies in the flue gas exiting thecatalyst regenerator section of fluid catalytic cracking (FCC) units.Current United States federal regulations limit particulate levels to 1lb. of solids per 1000 lb. of coke burned in the catalyst regenerator,or the equivalent of a flue gas particulate concentration ofapproximately 80-110 mg/Nm³. Corresponding European regulationscurrently vary considerably, from 80-500 mg/Nm³; however, this value isexpected to decline potentially to 50 mg/Nm³.

FCC technology, now more than 50 years old, has undergone continuousimprovement and remains the predominant source of gasoline production inmany refineries. This gasoline, as well as lighter products, is formedas the result of cracking heavier (i.e. higher molecular weight), lessvaluable hydrocarbon feed stocks such as gas oil. Although FCC is alarge and complex process involving many factors, a general outline ofthe technology is presented here in the context of its relation to thepresent invention.

In its most general form, the FCC process comprises a reactor that isclosely coupled with a catalyst regenerator, followed by downstreamhydrocarbon product separation. A major distinguishing feature of theprocess is the continuous fluidization and circulation of large amountsof catalyst having an average particle diameter of about 50-100 microns,equivalent in size and appearance to very fine sand. For every ton ofcracked product made, approximately 5 tons of catalyst are needed, hencethe considerable circulation requirements. Coupled with this need for alarge inventory and recycle of a small particle diameter catalyst is theongoing challenge to prevent this catalyst from exiting thereactor/regenerator system into effluent streams.

Overall, the use of cyclone separators internal to both the reactor andregenerator has provided over 99% separation efficiency of solidcatalyst. Typically, the regenerator includes first and second (orprimary and secondary) stage separators for the purpose of preventingcatalyst contamination of the regenerator flue gas, which is essentiallythe resulting combustion product of catalyst coke in air. Whilenormal-sized catalyst particles are effectively removed in the internalregenerator cyclones, fines material (generally catalyst fragmentssmaller than about 50 microns resulting from attrition and erosion inthe harsh, abrasive reactor/regenerator environment) is substantiallymore difficult to separate. As a result, the FCC flue gas will usuallycontain a particulate concentration in the range of about 200-1000mg/Nm³. This solids level can present difficulties related to either theapplicable legal emissions standards or the desire to recover power fromthe flue gas stream. In the latter case, the solids content in the FCCflue gas may be sufficient to damage turbine blades of an air blower tothe regenerator if such a power recovery scheme is indeed selected.

A further reduction in FCC flue gas fines loading is therefore oftenwarranted, and may be obtained from a third stage separator (TSS) devicecontaining a manifold of cyclones. Electrostatic precipitators are knownto be effective for this gas/solid separation but are far more costlythan a TSS, which relies on the induction of centripetal acceleration toa particle-laden gas stream, forcing the higher-density solids to theouter edges of a spinning vortex. To be efficient, a cyclone separatorfor an FCC flue gas effluent will normally contain many, perhaps 100,small individual cylindrical cyclone bodies installed within a singlevessel acting as a manifold. Tube sheets affixing the upper and lowerends of the cyclones act to distribute contaminated gas to the cycloneinlets and also to divide the region within the vessel into sections forcollecting the separated gas and solid phases.

In the area of cyclone design, significant emphasis has been placed onso-called “reverse flow” types where incoming gas is added around a gasoutlet tube extending from the inlet side of a cylindrical cyclone body.Particle-rich gas can be withdrawn from openings in the sidewall of thecyclone body, while clean gas essentially reverses flow from its initialpath toward the end of the cyclone body opposite the gas inlet, backtoward the gas outlet. The gas outlet is a tube normally concentricwith, and located within the cyclone body. These types of cyclones aredescribed in U.S. Pat. No. 5,514,271 and U.S. Pat. No. 5,372,707, wherethe inventive subject matter is focused on the shape and distribution ofthe sidewall openings in order to minimize turbulent eddy formation thatcan re-entrain solids into the clean gas outlet. In U.S. Pat. No.5,643,537 and parent U.S. Pat. No. 5,538,696, devices are contemplatedfor use with this fundamental cyclone design to further extend, orimprove the uniformity of, the vortex flow pattern and thereby increaseseparation efficiency.

Unfortunately, the requirement by itself for a gas stream to reversedirection and exit the cyclone body on the same side as the gas inletimposes flow disturbances that are not easily overcome. Cyclones of thetype described in U.S. Pat. No. 5,690,709, termed “uniflow”, eliminatethe re-entrainment of solids associated with the reversal of gasdirection. In this case, clean gas moves continually downward and exitsthe cyclone body below a lower tube sheet, which serves as the physicalboundary between the separated particles and purified gas. This design,however, also promotes non-uniform flow patterns, which are hereassociated with the discharge of particles at essentially right anglesto the particle-laden gas vortex, through the open bottom in thecylindrical cyclone body. Again, the basic operation of the cyclone inthis case involves a change in direction of gas flow that should ideallybe avoided. Furthermore, the open bottom design provides a relativelylarge surface area for exiting “dirty” gas to enter the bodies ofadjacent cyclones in an overall arrangement of cyclones, such as in aTSS. This communication of gas among cyclones reduces separationefficiency.

Aside from general considerations about cyclone design, such as theinduction of centripetal acceleration and the maintenance of a uniformflow pattern, further improvements in efficiency associated with anyparticular cyclone configuration must be verified through actualtesting. Indeed, some proposed designs that were believed in principleto mitigate uneven flow patterns and localized eddy formation actuallyperformed quite poorly in laboratory experiments. Even sophisticatedcomputational fluid dynamics computer software has been found in somecases to be a poor predictor of TSS separation efficiency. Therefore,through extensive trial and error, coupled with the overall objective ofrefining the cyclone internal flow pattern, a significant improvement infine particle separation from gas streams has been achieved.

SUMMARY OF THE INVENTION

The present invention is an improved cyclone for the separation of solidparticulates from a gas stream. Many of these cyclones can be combinedin a vessel for use as a third stage separator in the treatment ofsolid-contaminated gas streams, and in particular flue gas from arefinery fluid catalytic cracking unit or other solid-contaminated gasstreams. The cyclone provides a high separation efficiency because aparticulate-laden gas vortex is established and travels through thedevice with minimal flow pattern disturbances. The feed gas and exitingclean gas move in the same direction throughout the separation, and theclean gas, representing the bulk of the feed gas on a volume basis, isremoved from the central portion of the vortex using a gas outlet tubeextending with the cyclone body. Furthermore, solid particles are forcedthrough openings in the sidewall of the cyclone body to prevent backflowand gas communication among adjacent cyclones, rather than dischargedaxially.

The use of a plate or other structure to close off the bottom of cyclonebody means that particle-laden gas can exit only through openings on thecylinder wall. Thus, the pressure drop across the area through which thegas discharges is generally higher than that for open bottom designs.This increase in pressure drop and gas velocity induces a more forcefulejection of particulates through the cylinder sidewall, therebypreventing re-entry of solids into the cyclone body or any adjacentcyclones operating upon the same principal. In effect, the slots throughwhich the particle-contaminated gas exits act as a “check valve” toprevent backflow and particle re-entrainment into the cyclone body.

The cyclone of the present invention is effective for separating evenfine dust particles as small as 4-5 microns in diameter from the feedgas stream. These solid contaminants would otherwise render thecontaminated gas non-compliant with environmental regulations orpossibly prove detrimental to the proper functioning of power recoveryturbines.

Accordingly, in one embodiment the present invention is a cycloneseparator for location between an upper and a lower tube sheet in athird stage separator vessel. The cyclone comprises a substantiallyvertical cyclone body having a closed bottom end and a top end fixedwith respect to the upper tube sheet. The cyclone body defines a feedgas inlet at its top end for receiving a particle-contaminated gasstream from above the upper tube sheet. A sidewall of the cyclone bodydefines a plurality of discharge openings between the upper and thelower tube sheets for tangentially discharging particles and a minoramount of an underflow gas stream. The cyclone of the present inventionalso comprises one or more swirl vanes proximate the gas inlet to inducecentripetal acceleration of the particle-contaminated gas stream. Theapparatus further comprises a gas outlet tube located centrally withinthe cyclone body, extending through the closed bottom, and furtherextending through the lower tube sheet. The gas outlet tube defines aclean gas inlet, usually above the discharge openings, for receiving apurified gas stream from within the cyclone body and further defines aclean gas outlet located below the lower tube sheet for discharging thepurified gas stream.

In another embodiment the present invention is a fluidized catalyticcracking process for cracking a heavy hydrocarbon feed. The processcomprises contacting the heavy hydrocarbon feed with a cracking catalystto produce a light hydrocarbon product and a spent catalyst having cokedeposited thereon. The process further comprises regenerating the spentcatalyst in a catalyst regenerator by contacting the spent catalyst withair to burn the coke and provide a regenerated catalyst and a flue gas.The process further comprises separating the regenerated catalyst fromthe flue gas using a first stage and a second stage separator locatedwithin the catalyst regenerator to yield a catalyst fines-contaminatedflue gas stream. The process further comprises recycling the regeneratedcatalyst to the cracking reactor for further production of the lighthydrocarbon product. Lastly, the process comprises purifying thecatalyst fines-contaminated flue gas stream in a third stage separatorapparatus having an upper and a lower tube sheet contained therein and aplurality of cyclones between the upper and lower tube sheets. In thisembodiment, each cyclone comprises a substantially vertical cyclone bodyhaving a closed bottom end and a top end fixed with respect to the uppertube sheet. The cyclone body defines a feed gas inlet at its top end forreceiving a particle-contaminated gas stream from above the upper tubesheet. A sidewall of the cyclone body defines a plurality of dischargeopenings between the upper and the lower tube sheets for dischargingparticles and a minor amount of an underflow gas stream. The cyclone ofthe present invention also comprises one or more swirl vanes proximatethe gas inlet to induce centripetal acceleration of theparticle-contaminated gas stream. The apparatus further comprises a gasoutlet tube located centrally within the cyclone body, extending throughthe closed bottom, and further extending through the lower tube sheet.The gas outlet tube defines a clean gas inlet, usually located above thedischarge opening, for receiving a purified gas stream from within thecyclone body and further defines a clean gas outlet located below thelower tube sheet for discharging the purified gas stream.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified schematic view of an FCC unit of the prior art.

FIG. 2 is a simplified schematic view of a third stage separator of theprior art.

FIG. 3 is a cross sectional view of the cyclone of the presentinvention.

FIG. 4 is a sectional view of FIG. 3 taken along line AA.

FIG. 5 shows the improved separation performance efficiency of thecyclone of the present invention, compared to those of the prior art.

FIG. 6 shows the improvement associated with the present invention interms of its d50 value, or measure of the particle diameter for which50% removal would be obtained.

DETAILED DESCRIPTION OF THE INVENTION

The present invention applies to the purification of a broad range ofsolid-contaminated gas streams, and especially those containing dustparticles in the 1-10 μm range. A number of commercial gas purificationoperations meet this description, including the treatment of effluentstreams of solid catalyst fluidized bed processes, coal fired heaters,and power plants. Several well-known refinery operations rely onfluidized bed technology, such as a preferred embodiment of the processfor converting methanol to light olefins, as described in U.S. Pat. No.6,137,022, using a solid zeolitic catalyst composition. Another area ofparticular interest lies in the purification of fluid catalytic cracking(FCC) effluent streams that contain entrained catalyst particlesresulting from attrition, erosion, and/or abrasion under processconditions within the reactor.

As mentioned, fluid catalytic cracking (FCC) is a well-known oilrefinery operation relied upon in most cases for gasoline production.Process variables typically include a cracking reaction temperature of400-600° C. and a catalyst regeneration temperature of 500-900° C. Boththe cracking and regeneration occur at an absolute pressure below 5atmospheres. FIG. 1 represents a typical FCC process unit of the priorart, where a heavy hydrocarbon feed or raw oil in line 12 is contactedwith a newly regenerated catalyst entering from a regenerated catalyststandpipe 14. This contacting occurs along a narrow section extendingfrom the bottom of the reactor 10, known as the reactor riser 16. Heatfrom the catalyst vaporizes the oil, and the oil is thereafter crackedin the presence of the catalyst as both are transferred up the reactorriser into the reactor 10 itself, operating at a pressure somewhat lowerthan that of the riser 16. The cracked light hydrocarbon products arethereafter separated from the catalyst using first stage 18 and secondstage 20 internal reactor cyclones and exit the reactor 10 through line22 to subsequent fractionation operations. At this point, someinevitable side reactions occurring in the reactor riser 16 have leftdetrimental coke deposits on the catalyst that lower its activity. Thecatalyst is therefore referred to as being spent (or at least partiallyspent) and requires regeneration for further use. Spent catalyst, afterseparation from the hydrocarbon product, falls into a stripping section24 where steam is injected in line 26 to purge any residual hydrocarbonvapor. After the stripping operation, the spent catalyst is fed to thecatalyst regenerator 30 using a spent catalyst standpipe 32.

In the catalyst regenerator 30, a stream of air from line 34 isintroduced through an air distributor 28 to contact the spent catalyst,burn coke deposited thereon, and provide regenerated catalyst. Thecatalyst regeneration process adds a substantial amount of heat to thecatalyst, providing energy to offset the endothermic cracking reactionsoccurring in the reactor riser 16. Some fresh catalyst is added in line36 to the base of the regenerator 30 to replenish catalyst exiting thereactor as fines material or entrained particles. Catalyst and air flowupward together along the combustor riser 38 located within theregenerator 30 and, after regeneration (i.e. coke burn), are initiallyseparated by discharge through a “T” disengager 40, also within theregenerator 30. Finer separation of the regenerated catalyst and fluegas exiting the disengager 40 is achieved using first stage 44 andsecond stage 46 regenerator cyclone separators within the catalystregenerator 30. Regenerated catalyst is recycled back to the crackingreactor 10 through the regenerated catalyst standpipe 14. As a result ofthe coke burning, the flue gas vapors exiting at the top of theregenerator in line 42 contain CO₂ and H₂O, along with smaller amountsof other species. While the first stage 44 and second stage 46regenerator cyclone separators can remove the vast majority of theregenerated catalyst from the flue gas in line 42, fine catalystparticles, resulting mostly from attrition, invariably contaminate thiseffluent stream. The fines-contaminated flue gas therefore typicallycontains about 200-1000 mg/Nm³ of particulates, most of which are lessthan 50 microns in diameter. In view of this contamination level, andconsidering both environmental regulations as well as the option torecover power from the flue gas, the incentive to further purify theflue gas using a third stage separator (TSS) is significant.

A typical TSS of the prior art, containing numerous individual cyclones,is shown in FIG. 2. The TSS vessel 50 is normally lined with refractorymaterial 52 to reduce erosion of the metal surfaces by the entrainedcatalyst particles. The fines-contaminated flue gas from the FCCregenerator enters the top of the TSS at its inlet 54 above an uppertube sheet 56 that retains the top ends 58 of each cylindrical cyclonebody 62. The contaminated gas stream is then distributed among cyclonefeed gas inlets 60 and contacted with one or more swirl vanes 64proximate these inlets to induce centripetal acceleration of theparticle-contaminated gas. The swirl vanes are structures within thecyclone body that have the characteristic of restricting the passagewaythrough which incoming gas can flow, thereby accelerating the flowinggas stream. The swirl vanes also change the direction of thecontaminated gas stream to provide a helical or spiral formation of gasflow through the length of the cyclone body. This spinning motionimparted to the gas sends the higher-density solid phase toward the wallof the cyclone body 62.

The cyclone design shown in FIG. 2 represents the so-called “uniflow”apparatus where a bottom end 66 of the cyclone body 62 is open, allowingsolid particles that have been thrown near the wall of this cylinder tofall into the space 68 between the upper and lower tube sheets. Cleangas, flowing along the centerline of the cyclone body, passes through aninlet 70 of a gas outlet tube 72 before reaching the bottom end 66 ofthe cyclone body 62. The clean gas is then discharged via the gas outlettube 72 below a lower tube sheet 74. The combined clean gas stream,representing the bulk of the fines-contaminated flue gas, then exitsthrough a gas outlet 76 at the bottom of the TSS vessel 50. Theseparated particles and a minor amount (typically less than 10% of thefines-contaminated flue gas) of underflow gas are removed through aseparate particulate and underflow gas outlet 78 at the bottom of theTSS 50.

In FIG. 3, an individual cyclone separator 100 of the present invention,also affixed between an upper tube sheet 102 and a lower tube sheet 104,is shown. The cyclone 100 comprises an essentially vertical cyclone body106 having a closed bottom end 108 with the cyclone body fixed at itstop end 110 to the upper tube sheet 102. The closed bottom end 108 ispreferably in the form of a horizontal plate. The cyclone body defines afeed gas inlet 112 at its top end 110 for receiving aparticle-contaminated gas stream (e.g. a fines-contaminated flue gasstream) from above the upper tube sheet 102. Also, the cyclone bodyfurther defines a plurality of openings 114 for discharging gas. Theseopenings 114 are between the upper tube sheet 102 and the lower tubesheet 104, and are generally located in the lower portion of the cyclonebody 106. Preferably, these openings 114 are proximate the bottom end108 and extend upward from it. These openings allow for the discharge ofparticles along with a minor amount of an underflow gas, typically lessthan 10% of the particle-contaminated gas by volume, between the uppertube sheet 102 and the lower tube sheet 104. Closure of the bottom end108 induces a high gas velocity and pressure drop through the dischargeopenings 114 by providing relatively little surface over which theexiting gas can escape. This leads to an overall improved separation.

One or more swirl vanes 116 are located proximate the gas inlet at thetop of the cyclone to induce centripetal acceleration of theparticle-contaminated gas stream. A gas outlet tube 118 is locatedcentrally within the cyclone body 106, extends through the closed bottomend 108, and further extends upward through the lower tube sheet 104.The top and bottom ends of this gas outlet tube 118 define,respectively, a clean gas inlet 120 for receiving a purified gas streamfrom within the cyclone body 106 and near its centerline, and a cleangas outlet 122 below the lower tube sheet 104 for discharging thepurified gas stream. The clean gas inlet 120 is generally located abovethe discharge openings 114. The clean gas outlet 122 can be locatedanywhere below the bottom end 108. As mentioned, the cyclone body 106 isoriented generally vertically, so that separation of the solid phase isassisted by gravity. Preferably, the cyclone body is in the form of avertical cylinder, however, other shapes are certainly possible,including, for example, a cone shape.

As noted previously, the major advantage of this design is that itprovides a very uniform vortex of swirling gas that is essentiallyundisturbed along its downward path through the cyclone body and gasoutlet tube. A further advantage is related to the increased pressuredrop accompanying the ejection of particulate-rich gas through thecylinder wall openings. These openings provide a relatively smallsurface area for gas to exit, compared to the larger bottom ring-shapedsurface between the cyclone body and the gas outlet tube, used in theaforementioned uniflow cyclone designs. As a result, each openingprovides a type of “check valve” through which backflow of dischargedgas, a cause of reduced separation efficiency, is substantiallyeliminated.

The uniformity in gas flow is maintained in part through the use of aplurality of openings on the cyclone cylinder body for discharge ofparticles and a small amount of underflow gas. The openings may be ofvirtually any shape and located anywhere on the cyclone cylinder body,although it is preferred that at least some of these openings are nearthe closed bottom end of the cyclone to prevent an accumulation of solidparticles in this region. The openings may also be of varying shapes,for example, slots and holes, and located at various elevations on thecyclone body. Preferably, at least some of the openings are in the formof rectangular slots with their major dimension (length) substantiallyparallel to the axis of the cyclone body, as depicted in FIG. 3. Theseslots are normally spaced uniformly about the circumference of thecyclone body. Also, the vertical slot lengths usually range from about5% to about 25% of the length of the cyclone body. In a preferredembodiment, the lower ends of the rectangular slots are adjacent to theclosed bottom of the cyclone body.

To further promote flow uniformity and thereby improve overall solid-gasseparation efficiency, that the gas discharge openings are inclined froma radial direction. This allows gas to exit the cyclone body without asubstantial change in its swirling, tangential flow direction, asestablished within the cyclone body. An example of this desiredconfiguration is illustrated in FIG. 4, where the slots 114 also haveedges 200 that are beveled (i.e. not normal to the line tangent to thecircular cross section of the cyclone body 106 where the slots 114 arelocated). This beveling with respect to the curvature of the cyclonebody 106 has the desired effect of allowing gas to exit the cyclone body106 with a significant tangential velocity component and minimal changefrom the direction of gas flow within the cyclone body. Also, theleading edge along the principal length of each rectangular slot may beslightly raised from the general curvature of the cyclone body to divertthe gas flow in the desired tangential direction. Alternatively orconcurrently, the trailing edge of the slot may be sunk into the generalcurvature for a similar effect.

Furthermore, it has been determined that good solid/gas separationefficiencies are obtained when the openings are located below the cleangas inlet, which is also represented in FIG. 3. The total open areathrough which spinning gas may be discharged is preferably from about0.05% to about 5% of the surface area of the cyclone body. Thisparameter, of course, depends on several factors including solidcontaminant concentration, average particle size, gas flow rate, andpressure. When multiple cyclones of the present invention are used inthe design of a third stage separator (TSS) for an FCC refinery unit,the separator performance efficiency preferably includes a d50 particlesize of below 5 microns. As understood in the art, the d50 valuerepresents the diameter of a dust particle that is 50% removed in theunderflow gas of the TSS. Accordingly, in a preferred embodiment, thepurified gas stream has a concentration of particles of 5 microns orgreater that is less than about 50% of the concentration of particles of5 microns or greater in the catalyst fines-contaminated flue gas stream.

The performance benefit obtained using the cyclone of the presentinvention is further clarified in the following examples, which providelaboratory test data from experiments designed to simulate conditionsfound in FCC flue gas effluent streams. Although the following examplesillustrate specific embodiments of the cyclone separator of the presentinvention, they are not intended to limit the overall scope of theinvention as set forth in the claims.

COMPARATIVE EXAMPLES 1-7

The previously mentioned “uniflow” type cyclone separators of the priorart were compared in performance to various cyclone separators accordingto the present invention. Separation of particulate matter of 40 micronsin diameter and smaller from a flowing gas stream was investigated. Thecyclone separator in each test included a 280 mm i.d. cylindrical bodywith a 130 mm gas outlet tube concentric with the cylinder and extendingfrom about 250 mm above, to well below, the bottom of the cylinder.

In the comparative tests, except for this gas outlet tube extension, thebottom of the cylinder was open, although a disk was mounted on theexterior of the gas outlet tube about 130 mm below the cylinder bottom.Separated particulates, having been discharged at essentially rightangles to the spinning feed gas flow, were collected, along with a minoramount of underflow gas, in a dust hopper surrounding the cylindricalcyclone body. Both this gas and the clean (overflow) gas exiting throughthe gas outlet tube were analyzed for solid contamination levels as wellas the particle size distribution of these contaminants. Likewise, theseanalyses were performed on the feed gas.

In each separate experiment, the feed gas inlet flow rate to the cyclonewas maintained at 0.45-0.50 Nm³/sec. This gas contained 300-400 mg/Nm³of solids with a median particle diameter of 10-20 microns. Afterexiting the swirl vanes near the gas inlet, the gas velocity gas wasaccelerated due to the flow restriction effected by these vanes. The gasdischarged with the bulk of separated solids, called the underflow gas,represented either 1% or 3% by volume of the feed gas, depending on thespecific test. After each test, the efficiency of solid particulateremoval was calculated as a weight percentage of the feed solids thatwere removed in the underflow gas. The percentage of solid particles inthis stream of less than 10 microns in diameter was also determined,along with the calculated estimate of the particle diameter for which50% removal would be achieved (the d50 value).

Results for these comparative examples are summarized in Table 1.

TABLE 1 Underflow Vane Exit Separation Particles d 50 Comparative GasGas Velocity Efficiency <10 μm Value Example (vol-%) (m/sec) (%) (%)(μm) 1 1 24.1 76.3 78.5 7.5 2 1 39.3 82.8 84.8 5.7 3 3 40.5 82.1 93.96.2 4 3 40.3 83.7 93.3 5.5 5 3 24.1 80.7 76.8 6.7 6 3 38.9 84.9 87.5 5.57 3 39.5 85.0 87.4 5.4

EXAMPLES 8-12

The cyclone separator of the present invention was tested, by includingin the cyclone design a horizontal base that was used to close thebottom of the cylinder body. In accordance with the description of thepresent invention, the solid particulates were in this case dischargedfrom the spinning feed gas through openings in the cyclone cylindersidewall. This was achieved by forming two rectangular slots of about 90mm in length and about 10 mm in width. The length was parallel to theaxis of the cylindrical cyclone body, and the lower width dimension wasadjacent to the horizontal base closing the bottom of the cyclone body.The conditions of the feed gas flow rate, particulate level, and averageparticulate diameter were maintained within the ranges given in theComparative Examples. Again, studies were performed using underflowvalues of 1% and 3% by volume. Also, the same performance parameterswere evaluated and are given in Table 2.

TABLE 2 Underflow Vane Exit Separation Particles d50 Gas Gas VelocityEfficiency <10 μm Value Example (vol-%) (m/sec) (%) (%) (μm) 8 1 39.788.7 91.6 5.1 9 1 24.1 85.0 89.5 6.2 10 3 40.2 89.9 71.5 4.4 11 3 40.290.1 87.7 5.0 12 3 24.6 87.7 87.8 5.7

From the above test results, it is evident that the cyclone of thepresent invention, when compared to the open-bottom “uniflow” cyclone ofthe prior art, provides greater efficiency of solid particulate removalat both the 1% and 3% underflow conditions. This is illustratedgraphically in FIG. 5. Furthermore, the cyclone separator of the presentinvention is superior for removing particulates of 4-5 microns indiameter, which are relevant for the overall improvement of FCC thirdstage separator designs. The increased ability of the present inventioncyclone separator to separate small particulates, based on its d50performance parameter, is illustrated in FIG. 6. Lastly, in contrast tothe results in the Comparative Examples for cyclone separators of theprior art, the cyclone separator of the present invention consistentlyachieved a clean (overflow) gas solids contamination level of less than50 mg/Nm³, in compliance with current and even potential futurelegislation.

What is claimed is:
 1. A cyclone separator for use between an upper anda lower tube sheet, the cyclone comprising: a) a substantially verticalcyclone body having a closed bottom end and a top end fixed with respectto the upper tube sheet, the cyclone body defining a feed gas inlet atits top end, the feed gas inlet extending above the upper tube sheet forreceiving a particle-contaminated gas stream therefrom, the cyclone bodyfurther defining a sidewall of the cyclone body, the sidewall defining aplurality of discharge openings located between the upper and the lowertube sheets for discharging particles and a minor amount of an underflowgas stream; b) one or more swirl vanes located proximate the gas inletto induce centripetal acceleration of the particle-contaminated gasstream; c) a gas outlet tube defining a clean gas inlet in a top end ofsaid gas outlet tube, said clean gas inlet located centrally within thecyclone body for receiving a purified gas stream and said gas outlettube further defining a clean gas outlet located below the lower tubesheet for discharging the purified gas stream, the gas outlet tubeextending through the closed bottom end of the cyclone body and furtherextending through the lower tube sheet.
 2. The cyclone separator ofclaim 1 where the discharge openings have a total open area from about0.05% to about 5% of the surface area of the cyclone body.
 3. Thecyclone separator of claim 1 where the cyclone body is substantiallycylindrical in shape.
 4. The cyclone separator of claim 1 where theclosed bottom end is a substantially horizontal planar surface.
 5. Thecyclone separator of claim 1 where the clean gas inlet is located abovethe discharge openings.
 6. The cyclone separator of claim 1 where thedischarge openings are inclined from the radial direction to allow thedischarge of gas without substantial change from its tangentialdirection within the cyclone body.
 7. The cyclone separator of claim 1where the discharge openings define substantially rectangular slotshaving lengths substantially parallel to the axis of the cyclone body,the slots spaced uniformly about the circumference of the cyclone body.8. The cyclone separator of claim 7 where the vertical slot lengths arefrom about 5% to about 25% of the length of the cyclone body.
 9. Thecyclone separator of claim 7 where the lower ends of the rectangularslots are adjacent to the closed bottom of the cyclone body.
 10. Thecyclone separator of claim 7 where the lower ends of the rectangularslots extend to the closed bottom of the cyclone body.
 11. A process forpurifying a solid-contaminated gas stream using the cyclone separator ofclaim
 1. 12. A cyclone separator for use between an upper and a lowertube sheet, the cyclone comprising: a) a substantially vertical cyclonebody having a closed bottom end and a top end fixed with respect to theupper tube sheet, the cyclone body defining a feed gas inlet at its topend, the feed gas inlet extending above the upper tube sheet forreceiving a particle-contaminated gas stream therefrom, the cyclone bodyfurther defining a sidewall of the cyclone body, the sidewall defining aplurality of discharge openings located between the upper and the lowertube sheets for discharging particles and a minor amount of an underflowgas stream, said discharge openings having a total open area from about0.05% to about 5% of the surface area of the cyclone body; b) one ormore swirl vanes located proximate the gas inlet to induce centripetalacceleration of the particle-contaminated gas stream; c) a gas outlettube defining a clean gas inlet end located centrally within the cyclonebody for receiving a purified gas stream and further defining a cleangas outlet located below the lower tube sheet for discharging thepurified gas stream, the gas outlet tube extending through the closedbottom end of the cyclone body and further extending through the lowertube sheet.
 13. A cyclone separator for use between an upper and a lowertube sheet the cyclone comprising: a) a substantially vertical cyclonebody having a closed bottom end and a top end fixed with respect to theupper tube sheet, the cyclone body defining a feed gas inlet at its topend, the feed gas inlet extending above the upper tube sheet forreceiving a particle-contaminated gas stream therefrom, the cyclone bodyfurther defining a sidewall of the cyclone body, the sidewall defining aplurality of discharge openings located between the upper and the lowertube sheets for discharging particles and a minor amount of an underflowgas stream, said discharge openings define substantially rectangularslots having lengths substantially parallel to the axis of the cyclonebody, the slots spaced uniformly about the circumference of the cyclonebody; b) one or more swirl vanes located proximate the gas inlet toinduce centripetal acceleration of the particle-contaminated gas stream;c) a gas outlet tube defining a clean gas inlet end located centrallywithin the cyclone body for receiving a purified gas stream and furtherdefining a clean gas outlet located below the lower tube sheet fordischarging the purified gas stream, the gas outlet tube extendingthrough the closed bottom end of the cyclone body and further extendingthrough the lower tube sheet.